The work function of a Ni fully silicide gate or other metal can be tuned by adding alloy element to Ni target for Ni sputter deposition. The work function being the minimum energy needed to remove an electron from the Fermi level in a metal to a point at infinite distance away outside the surface. The work function is generally about half the ionization energy of a free atom of the metal.
The alloy element in Ni silicide is concentrated in the surface after silicide formation. However, the work function of a fully silicide gate is determined at the silicide/dielectric interface or the bottom of the silicide. Therefore, alloy element has little influence on the work function. Though alloy element can be redistributed uniformly in the silicide by high temperature annealing as known in the art, there is not effective way to drive alloy element to the bottom.
In order to enhance the alloy element effects, larger amount of alloy element is added into Ni target. However, when larger amounts of alloy element are added into Ni target, several issues occur, such as a phase separation of the microstructure in the target, which will impact sputter uniformity as well as increasing the possibility of thermal-stress cracking caused by thermal expansion misfits among brittle silicide phases. Also, the maximum amount of element is limited by Ni-alloy mutual solubility. Hence, the work function of a Ni-alloy silicide gate is thus limited.
Often, a different type of metal is desired or a different amount of silicidation is desired in order to create varying work functions dependent upon the device and its characteristics. Thus, there is a need for a silicided structure in which characteristics may be tuned or optimized for a particular application.